Influenza infection in pigs was first reported in 1918 and the first swine influenza viruses were isolated from pigs in 1930 (Shope, R. E., 1931, J. Exp. Med. 54:373-385). Swine influenza (SI) is an acute respiratory disease of swine caused by type A and C influenza viruses. Its severity depends on many factors, including host age, virus strain, and secondary infections (Easterday, 1980, Philos Trans R Soc LondB Biol Sci 288:433-7). Before 1998, mainly “classical” H1N1 SI viruses (SIV) were isolated from swine in the United States (Kida et al, 1994, J Gen Virol 75:2183-8; Scholtissek, 1994, Eur J Epidemiol 10:455-8; Olsen et al, 2000, Arch Virol. 145:1399-419). In 1998, SIVs of the subtype H3N2 were isolated in multiple states in the United States.
SIV replication is limited to epithelial cells of the upper and lower respiratory tract of pigs, the nasal mucosa, ethmoid, tonsils, trachea, and lungs, and virus excretion and transmission occur exclusively via the respiratory route. Infectious virus can thus be isolated from the tissues mentioned, as well as from tonsils, bronchoalveolar lavage (BAL) fluid, and nasal, tonsillar, or oropharyngeal swabs (Kristien Van Reeth and Wenjun Ma, 2013, Current Topics in Microbiology and Immunology 370: 173-200).
The influenza virions consist of an internal ribonucleoprotein core (a helical nucleocapsid) containing the single-stranded RNA genome, and an outer lipoprotein envelope lined inside by a matrix protein (M1). The segmented genome of influenza A virus consists of eight molecules of linear, negative polarity, single-stranded RNAs which encode eleven polypeptides, including: the RNA-dependent RNA polymerase proteins (PB2, PB1 and PA) and nucleoprotein (NP) which form the nucleocapsid; the matrix membrane proteins (M1, M2); two surface glycoproteins which project from the lipid containing envelope: hemagglutinin (HA) and neuraminidase (NA); the nonstructural protein (NS1), nuclear export protein (NEP); and the proapoptotic factor PB1-F2.
The type A influenza viruses are divided into 17 H (hemagglutinin) and 10 N (Neuraminidase) subtypes which can give rise to many possible combinations (designated as H1N1, H1N2, H2N1, H2N2, H5N1, H5N2 and so on) (Tong et al., 2012, Proc. Natl. Acad. Sci. USA, 109: 4269-4274). The hemagglutinin (HA) plays role in attachment of the virus to the surface of infected cells while the neuraminidase (NA) plays role in release of the progeny viruses from the infected cells therefore NA plays role in spread of the virus (Wang et al., 2009, Biochem. Biophys. Res. Commun., 386: 432-436).
The pathogenicity of influenza viruses is modulated by multiple virus and host factors. Among the host factors that fight virus infections, the type I interferon (IFN[alpha]/[beta]) system represents a powerful antiviral innate defense mechanism which was established relatively early in the evolution of eukaryotic organisms (Garcia-Sastre, 2002, Microbes Infect 4:647-55). Influenza A viruses express a non-structural protein in infected cells, the NS1 protein which counteracts the cellular IFN[alpha]/[beta] response (Garcia-Sastre et al., 1998, Virology 252:324-30).
Modification of the NS1 can be utilized to produce live attenuated SIVs as described by Solórzano et al. 2005 (J Virol 79:7535-7543), Vincent et al 2012 (Journal of Virology 19: 10597 to 10605) and in WO 2006/083286 A2. Attenuated SIVs expressing NS1-truncated proteins of an H3N2 SIV (sw/Texas/4199-2/98, Tx/98) with 73, 99, or 126 amino acids (Tx/98 NS1D73, Tx/98 NS1D99, and Tx/98 NS1D126) have been generated using reverse genetics.
However, commercial vaccines currently available against swine influenza virus (SIV) are inactivated, adjuvanted, whole virus vaccines, based on H1N1 and/or H3N2 and/or H1N2 SIVs for intramuscular injection (Kristien Van Reeth and Wenjun Ma, 2013, Current Topics in Microbiology and Immunology 370: 173-200). In sow herds with high antibody levels to SIV from either vaccination and/or natural infection, vaccination of piglets should be delayed until the age of 12-16 weeks to avoid interference with maternally derived antibodies (MDA).
There are two major weaknesses of the immune response induced by killed virus vaccines. First, such vaccines induce only serum antibodies, no mucosal antibodies and, further, the vaccine induced serum HI antibody titers decline rapidly between 2 and 6 weeks after the booster vaccination. Second, inactivated vaccines in general do not enter the endogenous pathway of antigen presentation and are unable to activate virus-specific CD8+ T cells or a CTL response.
Further, when combining different antigens for generating a combination vaccine the efficacy of the single components may be affected by a phenomenon called interference.
A further problem arises as most SIV vaccines are used for sow vaccination resulting in long lasting maternal SIV antibodies in piglets (Markowska-Daniel et al., 2011, Veterinary Immunology and Immunopathology, 142: 81-86). Regarding this, vaccination of piglets may be difficult to combine with vaccination of sows because of prolonged maternal immunity.
Thus, there is a need for SIV vaccines being highly efficacious and administrable early in age.